This invention relates to a jet engine nacelle which is aerodynamically influenced in its entry lip area by flaps.
Modern gas turbine jet engines for use in long- but also short- and/or medium-haul aircraft primarily are constructed in the form of turbofan or ducted-fan engines of multi-spool construction, where the predominant portion of propulsion is provided by front or aft fans or compressors. With these engine types a free power turbine downstream of the compressor turbine of the gas generator (inner or first engine cycle) is often used to drive the fan sustaining the outer or second engine cycle.
These engines are installed in nacelles which form a so-called annular "intake lip" at the leading end. With a view to sustained or cruise flight operation, which for the type of aircraft exemplified above is the most commonly used mode, it is generally attempted to attune the geometry of the intake lip to sustained or cruise flight conditions for minimum aerodynamic losses with respect to ambient and intake air flows. Further, in the interest of a relatively small engine diameter, e.g., it is attempted to keep the intake lip area relatively slim and optimal from the consumption aspect.
While this geometrical design of the intake lip may be advantageous from the sustained or cruise-flight aspect, it completely ignores two crucial cases. A first crucial case is when an aircraft is in a take-off phase in which the aircraft is directly about to lift off the take-off strip and the load on the engine is relatively severe and the air and gas mass flows through the engine are high. At this time, the aircraft is inclined at a considerable local angle of attack with the take-off strip relative to the ambient air flow and a risk exists of notable flow separation of intake air portions on the radially inner wall area of part of the lower intake lip. Thus, irregularities of the flow at the engine inlet and indifferent vibration and flutter tendencies may be caused on the blower or fan blades, and even the risk of gas generator compressor surge cannot be excluded from consideration as long as the separation region propagates far enough towards the gas generator (compressor).
The second crucial case is that of possible failure of a jet engine on, e.g., a twin-engine aircraft, when the aircraft is inclined at a relatively steep angle with respect to a horizontal plane and when an only remaining, extremely low airflow through the engine may cause autorotation (windmilling) of the engine, a condition that is separate from the normal aero-thermodynamic cycle. In this second flight case, a risk exists of relatively pronounced flow separations of the air on the outer wall of the intake lip and over a radially outer circumferential sector of the latter. Engine failure of this type is considered critical especially with twin-engine aircraft, for the reason that the grave flow separations in the lip region of the nacelle outer contour make it impossible to maintain the prescribed gradient of climb.
In order to prevent said first grave case and its consequences from developing, air injection flaps in the form of so-called blow-in doors had previously been proposed that were to permit a locally variable intake lip geometry designed such that the desired local fineness ratio of the intake lip was ensured during cruise, while on the other hand, the flow separations forming radially inward on the lower portion of the intake lip were eliminated by air injection effected from the outside to the inside into the intake air flow.
Said air injection measures, however, contribute nothing towards solution of the problems associated with the second crucial case (failure of one engine). Fixed radially outer expansion of the lip structure on the nacelle outer side, which might conceivably be used to cover this relatively rare flight condition, would not only be associated with a nacelle weight penalty, but also with an enlarged air-wetted surface area of the blower cowling formed by the nacelle, aggravating the friction and form drag of the nacelle; and all of these consequences harmonize little with a nacelle fineness ratio that has been aerodynamically optimized for sustained or cruise flight.
In a broad aspect of the present invention, a nacelle of the initially cited generic category is provided which eliminates the suction peaks and flow separations occurring especially with a view to engine failure on an upper circumferential sector of the intake lip, on the outside, and which makes it possible to achieve a suitably slim geometry of the nacelle intake lip to meet cruise or sustained flight requirements.
In especially preferred embodiments of the present invention, the above-noted objects are achieved by providing flaps on a radially outer circumferential sector of the entry lip, which flaps are movable between a cruise flight position in conformance with the contours of the nacelle and an extreme position in which they externally expand the entry lip radially and simultaneously provide exposed blow-off ducts extending essentially tangentially to the outer wall of the nacelle.
In accordance with preferred embodiments of the present invention, the lip outer contour can be expanded radially outward in flap-like fashion to the geometric extent desired exclusively for said critical case (engine failure/windmilling), without having to expect an appreciable amount of overflow of the airstream forming on the leading edge of the intake lip that would cause said suction peaks and flow separations.
The blow-off ducts formed in connection with a second extreme position of the flaps prevent a stagnation zone of the air stream from forming downstream of the respective outermost flap extension (outer flap), so that this arrangement on balance provides a rather low-drag blow-off configuration.
The solution according to the invention also eliminates the need of continuing with a fixedly arranged local expansion of the wall on the radially outer side of the intake lip also in cruise or sustained flight operation. This facilitates lower weight of the nacelle, and it considerably reduces the aerodynamic drag of the nacelle for improved economy of the aircraft.
In accordance with especially preferred embodiments of the invention, inner and outer flaps are arranged for automatic pivotal movement to permit air blow-off in response to the differential pressure between the static pressure on the outside and the static pressure on the inside of the lip area affected. Arrangements are also contemplated utilizing a suitably combined arrangement of air injection flaps to optimize the utilization of the available space at the nacelle. In especially preferred embodiments, a generally aerodynamically optimized, slim engine nacelle of low aerodynamic frontal drag (cruise flight) is obtained while providing solutions in said two, initially cited crucial conditions.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.